If you want to make such an ambitious project make yourself a name in the community first. It's a lot easier to get people to join if they know who you are and that you have the ability to lead not only a small line of 12 people but have been consistently and long lasting in your projects. There are some armies around as far as I know, e.g. the IVe Corps, a french speaking Corps of multiple regiments (Line, Grens, Skirms, Cav and Arty). They have been around for 3 years already I believe - it is possible to form something like that.
Anyways good luck with your endeavours.
The difference is that the IVe was, at the beggining, only one regiment, and it opened new regiments with the time, and the recruits that joined allowed us to grow 5 regiments of 5 different units.
The project, here, consists in gathering some already existing regiments, with their leaders etc, and that is way different. To be fully honest I don't think that kind of thing is still possible nowadays, the personal ambitions of the different leaders will fuck up the whole thing. You need to pick people from your own ranks, but for that, one needs quite a lot of members, and yeah, that's hard.