Does it matter what web 2.0 I'm using

Some web 2.0 have different main topical trust flow. For e.g Tumblr has computer as main topical TF, while blogpost has society.

Since I'm using a subdomain of that main domain, does it matter what topical TF the root has?

Also, does it matter the strength of that particular web 2.0's root, since subdomains are considered "fresh"?

Topical TF doesn't matter.

Start by using the web 2.0s with highest DA first.

I have heard that expired tumblr's are quite good. They usually have links pointing to them so have some additional authority, although it would be best to check the link profile first.

I think you should use as many different web 2.0 as you can to, not leave a footprint.