Two tanks & three growbeds, what is the best configuration?

[Ron Faulkner]

The other day I managed to obtain three 50 gallon cold water tanks.  I'm pretty sure that I want to use three of them for growbeds and two as fish tanks.  

1). Should I connect them together for convenience in pumping?

2). Should I have different species in each tank?

OR use one as a breeding tank?

50 Litres will only support 8 or 10 fish and that's not really enough for me.

Gravel,

 Is 'pea gravel' ok or is it too small for the growbeds? it's typically 6mm diameter.

No doubt I will have more questions as the building and cycling proceeds.

Any guidance much appreciated at this stage.

Thanks

PS I live in Devon UK, so I think trout is the easiest fish to start with for our cold winters.