I don`t know why they replaced the Praetorian cavalry in RTR. Thats a historical unit, also it is out of RTWs timeframe. Tacitus mentioned them quite often, fighting in the 69ad civil war. The praetorian cohorts were probably organized like the cohortes equitatae of the auxilia. Units with ca. 70% infantry and 30% cavalry.

The equites singulares of the empire are also out of the timeframe. The best horsemen of the province cavalry units were attached to them temporaly.

I don`t know their actual size, but i think that the normal general`s bodyguard is sufficient. In RTW all units are at least 3 times too small. A generals bodyguard unit has 50-100 soldiers, so it should be enough