Of course. PocketMine will always work with plugins that are made for it, unless the plugin just has a ton of problems.
Unless the API version hasn't been tagged, released and finalised. The current ALPHA-8 API is subject to change at anytime as it is in-dev. As me, @SOFe and @dktapps have said before: you're better off using the tagged releases on github so your plugins aren't constantly breaking.
If that's what plugins are made for, or if there old plugins that were originally for whatever API you're using, than you shouldn't have to worry. Steadfast is based on a really old version of PocketMine, but has been reworked to work with newer versions of MCPE. Some count it as a "spoon", but I think @dktapps says differently (that is if I remember correctly) because of it's differences (correct me if I'm wrong)
One thing to note is that steadfast is not really for ordinary servers. If you expect to use any public plugins, steadfast is not designed for you.
I know. It's made for completely custom mini game servers, and is made by a company that owns a network of them.
Also note that this forum is not for Steadfast. Even though Steadfast is or is not technically a spoon, you will still get warned for "Spoon support" if you continue to ask for help with Steadfast