Hi @jerash!
First, please, it’s better you don’t call “plugins” to standalone engines, as it creates confussion. Zynthian supports LV2 plugins through MOD-UI, so integrating LV2 plugins into Zynthian is pretty straightforward: only download and compile, creating an “install recipe” BTW 
Integrating standalone engines is a little bit more complex, althought not too much. You only have to implement a zynthian_engine class. Take a look to “zyngine/zynthian_engine.py” and so on …
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Aeolus: is a very interesting “Church Organ” emulator. There is no LV2 version, so it should be integrated as standalone engine. I’ve no experience with aeolus, but a basic integration should be done in a few hours.
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Linuxsampler: the current version included with zynthian should be upgraded ASAP. It would be nice if you or someone else try to do it and write an “install recipe”. If you do so, remember to first remove the current version, that is installed from repository, so:
# apt-get remove linuxsampler
and after that, download, build and install the new version
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Bristol Synths: Some time ago i studed this synth collection for integrating into Zynthian, but i found that the project is completely abandoned and the sound quality of the emulations is very low. Also, the code is quite unstable, and fail very often. But the skins are really beautiful, yes
Kind Regards!