I think you'll find that CAA Aerodrome Standards Division lays down minimum lateral separation criteria for simultaneous F/W and rotary movements, but these criteria do leave a certain leeway for interpretation. I can think of one airfield where they were quite strictly applied, although up until then the common-sense approach (no pun intended) had worked well for many years without incident, other than the occasional straying of aircraft off their respective centrelines during climb-out.
As for downwash effects, the R22 or similar sized Schweizer/Hughes helos are unlikely to affect manned light aeroplanes: not enough to cause them to do anything dramatic, like overturn. An unmanned aicraft may well have its control surfaces buffeted about, possibly resulting in damage. Nevertheless, a light aeroplane manned or unmanned should be avoided by as great a margin as possible: certainly at least one rotor span. Microlights should be given a very wide berth!