Clearing up storage, what's safe to transfer?

There may be a double-candy event during Easter, but it's not a guarantee, and in the event it doesn't happen, you'll be hoarding Dratinis for an unknown period of time. Just transfer the useless ones now. Same with everything else you're hoarding for double-candy transfer, really, aside from the really-rares (Beldum, Bagon et al) and Legendaries.

As for the uncommon critters, maybe just try to save three or four of each, tops, with the highest IVs. For instance, a L20 80IV Charmander (Swinub) is probably never going to see the light of meta-viability even if (when) Mega-Charizard (Mamoswine) enters the fray, given the high dust and candy costs involved in bringing it up to L30+. This should keep you going until Easter, and if the double-candy event does happen, you'll be able to clear more storage space through transfers then.

Honestly storage space is something I struggle with too, I think PoGo has enlightened me to the fact that I may be a hoarder, deep down.

Community Days with exclusive evolution only moves hurts trying to clear out unnecessary pokemon. Now I feel the need to hold onto good unevolved pokemon where before I would have just evolved for fun defenders or thrown out.

First off: There is definitely a limit to how many Dratini candy you can possibly need. Check that you will actually have a use for the additional candy you would gain from hoarding them and then consider how valuable is that extra candy compared to taking up your storage space for potentially months.

I'm inclined to say that yes, everything with low stats, no mega evos and no cross-gen evos is fairly safe to transfer. We could always get prestiging back, or we could get breeding with egg groups or any number of things that make bad pokemon useful. Doesn't make much sense to try to prepare for everything if short on space.

I have two 98% level 30 Furrets. They are my 6th and 7th highest CP Furrets thanks to weather boosting. I don’t think I can justify keeping them anymore.

I probably have 5 other similar situations (Noctowl, Xatu etc...)