Regarding how to play middle pairs at 25nl, I think it is much more important to consider the opponent rather than the cards.
Against an unknown both flatting and 3betting have merits. But what is the purpose of the bet? Do you want a Call or a Fold?
More info on villain makes it easier.
vs wide ranges and high Ft3B I'd raise smaller pairs as I likely have the best hand and my hand doesn't play so well post flop. JJ plays okay post flop and I have position so flatting makes more sense.
Against wide ranges and low Ft3B flatting small pairs makes more sense as I have little fold equity, so I set mine. But 3betting JJ makes more sense because he will call with worse.
It's also useful to see how aggro they are post. The more passive they are the easier it is to get 99 to a cheap showdown.
If they're playing reg/abc poker then mixing it up a little depending on table dynamics and any history is a good idea i.e. don't 3bet 99 if you've already 3bet a ton of hands on this table, but if you've been card dead then 3betting with 99 is a good metagame play.
77-JJ are all hands that can be 3bet for value, but it is situation specific. I'd avoid having any fixed strategy with these cards.
I've followed Pleno's advice and put the PFR by position on my HUD and find it very useful for making decisions with middle pairs.