Domain Name - MicroNiche Site Question =/

Allright so I have a general question on micro-niches and their respective domain names. I understand that trying to get the exact keyword your trying to rank for as your domain name is best for ranking purposes but lets face it, alot of the time most of the top keyword domain names are already taken. So, would you either try to get your exact match domain name with a lesser ranking keyword or, would you rather try and develop a brand name as your domain name?

That was the first question, my second question is..

Say your in the 'babies' niche and you want to create several micro-niche sites, one targeting baby strollers, one targeting baby cribs, etc. and you wanted to go with the brand name domain instead of a keyword domain. Would you still get seperate domains for each such as, strollerheaven.com, babycribcrabby.com (just examples), etc. orrrr, would you go with a certain brand name domain that you can use for all your future sites (or is this not even possible?) Such as, would you purchase the domain, babytastic.com and then create all your mini-niche sites as subdomain sites, such as purchasebabycribsonline.babytastic.com
buybabystrollersformybaby.babytastic.com??

I'm not sure if this is a silly question - my lines are just blurry right now as to what is and isn't correct and I don't wish to purchase any domains until I'm clear on this matter so thank you so so soooo much for any help in clearing my understanding!