Although I'm not well-versed in rescue organizations per se, I agree with taking in a limited number rather than a limit on size. Although, it might be plausible to sort of lean toward the tinies that have stolen your heart as time goes on and you'd maybe become an "expert" in that area. I do a lot of volunteer work and am on the auxiliary (read: fundraising and supporting) one of the local animal SPCA shelters here. I think it's an EXCELLENT idea to start doing your homework at a local shelter. See what's involved in the day to day, month to month duties, care, supporting, maintaining etc. I'm sure you have an idea, but when you're responsible for a bunch (regardless of how many...if they're really sick/injured...there's SO MANY things that can be wrong with them) ...do they need to be isolated? If they do, you need to have the policies and procedures in place, not to mention, an area that they can be isloated from another animals, including their food and dishes; and you sure don't want to be exposing your own dogs to contagious diseases, whatever it might be. Sanitation is another big issue too.
My one thought about not just being strictly tinies, is that if you have several of them that are really needy, you get really exhausted really quick. Kind of like how they (should) assign a patient load in the hospital; you might have one super super sick patient, but the rest are just moderately ill. Instead of having 'X' number of all super super sick patients. So, if you had some of the more normal sized Yorkies, you might have at least a few issues that you don't need to deal with.
And you have to keep in mind that you have your own pets that need and deserve your love and attention. I can tell you that I would LOVE to volunteer a LOT more of my time to walking the dogs, cleaning their crates and cages and just spending time petting or brushing them, but my first duty is to my OWN dog (and cats and birds) that I CHOSE to bring into my life and "love and cherish til death do us part" in my opinion....An hour or two a day I can spare, but more than that, I have other things that take me away from the animals too just in daily life; normal daily life type of stuff; and if it were a turtle or some fish who don't really give a hoot if I'm around all day, it would be different, but I just really think Yorkies need more than a passing pat on the head. And I'm not here to save the world; I'm here to do the best for what I've committed to bring into my life without shortchanging them in any way, just because I have a need to save and love everything (I'm not belittling YOU, I'm just saying that I might have a tendency to do that if I didnt' have more rational people around ME at times).
I think your heart is in the right place; but I can say almost without a doubt that your own pets, unless they're being cared for by someone else most of the time will suffer. I could read you chapter and verse about that b/c I've got a very good friend who is a perfect example.
I think just volunteering at an animal shelter and maybe doing some fundraising or volunteering for some of their fundraisers (contact their auxiliary) would give you some good experience and see what all is involved.
I'm certainly not saying it can't be done; and I can see you're just feeling your way around it....but this is my opinion and advice to you. I know a woman and her husband who started and maintain a wild animal rescue basically on their own; if you're interested in contacting them, PM me and I'll get their info for you. She was recently nominated for a "Hero of the Year" award on the Animal Planet. Actually, here's the story:
http://animal.discovery.com/converge...007/linda.html