Compatability between 195s and 331s is the same as between 171s and 377s and between 380s and 385s - mechanical compatibility to make rescue of one type by the other quicker and easier, but no electrical compatibility.
The mistake with the 195s wasn't so much that they're diesel only as there are still many routes that have little/no under the wires running, but rather that they have mechanical instead of electric transmission. With electric transmission conversion to bi-mode in the future would only have required the insertion of a pantograph/transformer vehicle and as the Civity UK design is still in production this wouldn't have had the issues that the abandoned Voyager bi-mode conversion project had.
A 195 (and for that matter a 196 or 197 too) can still be converted to bi-mode however it would be expensive and time consuming, but probably still cheaper than buying completely new units. In addition to inserting a pantograph/transformer vehicle, the current mechanical transmission would have to be removed and replaced with electric transmission, and the couplers would have to be changed to the 110v standard for EMUs.
With regards to the 80xs there are three groups of compatible subclasses, which includes bi-modes and electrics working together. 800/0+800/3+802/0+802/1 are compatible, 800/1+800/2+801 are compatible, and 802/2+802/3+803 are compatible. There has been an instance of an 802/2 working with an LNER 80x on an Eastleigh move, however I think software updates now make this impossible.
You're probably thinking of the FLIRT bi-modes. I'm fairly sure even the FLIRT DEMUs have omitted a pantograph well, which is utterly silly. Making structural alterations later is very difficult; even pretty much all third rail EMUs down South have pantograph wells for ease of later conversion to 25kV if needed.
231s have got pantograph wells and can quickly and easily be converted to bi-modes. They are effectively 756s with a higher top speed and without the pantograph.