Things to consider in selecting strings
Scale length : At the same pitch, longer the scale length higher the tension is and vise versa.
The scale length of many 19th century guitars are around 620mm, while that of most modern classical guitars are 650mm or longer. The latter use the resulting higher tension to increase volume, projection, and sustain of the instruments. Ramirez has a 664mm model, for example. Modern classical guitars tend to have deeper bodies and larger sound board area in recent years. It is my personal impression that an use of a slant bracing bar by Ramirez is meant more to withstand higher tension than to get better sound characteristics. By the way, a bracing scheme using a slant bar can be seen on many guitars besides Ramirez', though Jose seemed to have acquired a patent on it. Instruments that use multiple steel strings per course, like Gibson's mandolins and so called 12 string fork guitars, should be under extremely high tension. Strings by Gibson in average seem to be in a slightly higher tension than others.
String angle at saddle and nut : You have to be careful on the angle at the point where the strings are supported. Larger the bending angle of the string at saddle, greater the force exerted onto the sound board (or vise versa) becomes. Higher the string tension, more the volume tends to be generated by an instrument. At the same time, more force is needed to hold down the strings by left hand and to play them by right hand. A luthier told me that a 19th century guitar (he was talking about French style instrument) would be certainly damaged when tension of 8kg/string were used like modern classical guitars. It seemed better for a 19th century guitar to strung with a tension of 4~6 kg/string and to adjust such a way that it responds slight touch of both hands. You are to play an instrument that was used with high tension at the period, though, you have to string it accordingly. It often is the case that the appropriate tension of instruments depends on their type (locality, period, and structure), even when the scale lengths are the same.
Because shorter scale length results lower tension, there seems to be little problem using modern classical guitar strings with a typical scale length of 650mm on a 19th century guitar of 630mm. But, the strings seem to be too thick and tension feels rather high, if this is attempted. Many 19th century guitars use pins to affix strings at the bridge and higher saddle, which apply more pressure on the sound board than modern guitar with the same string tension. You have to be careful about the angle at the saddle.
The following two pictures show bridges of two 19th century guitars which are unaltered and in an original condition. I took these while adjusting saddles in repairing processes. The guitar on left has not much string bending at the saddle and one on right has a higher saddle which makes the bending angle larger. These are the differences that existed at the time they were made.
The next photo shows Spanish (or Portuguese) guitar made around 1820. The photo was taken while changing strings after a repair. A part of bridge structure acts as a saddle, therefore the saddle height and string angle at the saddle changes as the bridge wares down. Tension also should change slightly. It is quite similar to the bridge of modern classical guitars.
Bracing scheme : instruments with fan bracing seem to sound better with higher tension.
Many modern guitars adopt a fan bracing scheme, but 19th century guitars tend to use a few parallel bars. Some say that instruments with fan bracing by Panormo seem to sound better with relatively high tension. But, Panormo made a variety of instruments and there remains a few of his having a very thin sound board. I would say that it depends on the individual instrument. There exists some instruments with fan bracing scheme originated from Panormo that sound better with lower tension. Many recent classical and steel string guitars are made with complex bracing schemes, like Kasha bracing, which tend to be designed to withstand more tension. The body size tends to become larger with larger sound board area and thicker body.
Density of string : Heavier strings tend to generate low tone and lighter ones high.
Metal wound, fluorocarbon, gut, and nylon are typical string material in an order of density (high to low). One can use fluorocarbon lines sold as " Seaguer" from Kureha chemical in fishing supply shops. Nylon fishing lines can also be used on stringed instruments if they are proper gauge ones. Gut strings can be found at music shops specialized in early string instruments. Ground nylon strings and plastic wound strings for treble are sold by Savarez and others. ProArte by D'Addario and Augustine strings are popular among modern classical guitar players. Hannabach seems to use a different scale on string tension and their "normal tension" seems to be harder than some "hard tension" ones of other companies. As special cases to increase string density, there are twisted multiple gut strings, metal wire wound over gut strings, metal powder doped gut strings, etc. You can also find metal salt loaded gut strings distributed by early string makers. Among gut strings, those with extra twist to increase density are called "high twist". Among ukulele strings, there is a set of all wound strings for Dobro uku's.
Tuning pitch : Modern pitch is A=440Hz, but the standard pitch used in 19th century was A=430Hz (1/4 tone lower) or A=435Hz.
I used to use Hannabach's "Super low tension" in A=415Hz,
because this is exactly a half tone lower than modern pitch. But,
this tuning is not practical to play in an ensemble. Currently, I
usually use strings for early instruments in 440 Hz. It's common to
use A=440Hz and better to select strings for that pitch in an
ensemble. For an audience with an absolute pitch, it may be hard to
listen other than 440 Hz ;-P.
Playing with nail or finger tip : It is preferable to use higher tension for playing with nail and lower tension with finger tip.
This summarizes the tendency of many people's observations. Though, it's not an easy problem to answer, because personal preferences take a large part in judging playability, sound, and volume of an instrument. Personally, I use very short nail for playing single string (19th century, modern classical, or flamenco) and double string guitars, ukuleles, and steel string instruments, though not skilled enough to change touch depending on the instrument. Even period lute players had played either with nail or finger tip. Some references claim that lutenists do not use nail, but this is not true. However, finger tip playing may have some advantages on playing double stringed instruments.
1st string (course) tends to stretch easily : this is especially true for thin gauge nylon.
It seems better to use slightly thicker gauge string for 1st one than required. There is a side effect of a melody line being clearer with this.