That idea is fairly simple to create. I've seen the anime so I'll lend what help I can
Simply get tau fire warrior arms and cadian bodies, legs and heads. Boom, infantry. You can also just get the shoulder pads and the individual guns and just apply them to the guard models.
An image of a conversion:
For the suits, conversions will be necessary though using the forge world models might help with that, or at least hunting for bits.
Here is a farsight suit I found that somewhat matches:
Trying to make a code geass themed tau force is a very plausible idea. Just realize having some experience doing conversion work would be handy.