All those veg you mentioned are fine to grow together..they are classed as 'salad' stuff so no problems with competition in there. Fennel can grow bit larger so give it more room.
I would say with your size beds..one of two beds should yield plenty of 'salad stuff'' for several people so you have plenty of room to explore other veg too...how about some dwarf (bush) 'french' beans for half a bed and other half some 'container' cherry tomatoes..you could fit quite few plants in that space and yield loooads of small cherry tomatoes..that type of varieties are many listed on cataloques and shops.
And you still have room for much more...
As for the the bed filling..soil is good..too much compost will shrink away from the edges of the bed when it dry out and re-wetting can be bit trouble..so any of those crops mentioned added compost into beds is suitable..but mix it well into soil. You don't have to get the compost in deep as salad crops don't have very deep root system. They will benefit its presence in few top inches of the bed.
I would use soil as the 'main' incredient and think any compost/manure as 'soil conditioner'..bit like butter on slice of bread..to make it all that more nicer and palatable.. If you don't have access into home made-garden compost, shop bought will do. Any manure would need to be 'well rotted' as fresh will be too strong for many plants and result burning their roots before they have chance to grow...and with salad stuff that is eaten uncooked it is better not to use it nearby when freshly added anyway. Some pathogens may lurk in it to spoil you food. Once manure has been in ground for while it safe and don't do any harm. Potatoes will benefit with lot of well rotted manure..and by the time you dig spud up..manure has been in ground for months and you don't eat potatoes raw neither so there is no health issues involved.
Good luck with you 'venture..may the crops be plentiful and weather on your side this coming season..
Don't hesitate drop some more questions if you are not sure of something..once you've done it first time..there is no going back and it is much more easier there on. You've done the hard bit now with creating the growing space..enjoy!