Rules of thumb are that ice can form in cumulus anytime. In summer likely to be light in winter can be severe.
Not sure where that comes from. My impression is that there's a lot more ice around in summer, it's just higher up. The higher absolute humidity and greater vertical extent of cu in summer tend to create more problems. May and June tend to be worst, with greatest instability.
In stratus it's normally much less of an issue.
That's certainly true, though stratiform cloud can still catch you out. Last January I descended from clear blue into a flat layer of SC between about 2500 and 4500 ft. At 5000 ft the temperatures were positive, so I blithely assumed the SC would be ice-free. ATC stopped my descent at 4000 ft, the temp fell to -2 degC and the ice went on faster than I can remember in stratiform cloud. No emergency, particularly with boots and prop-deice, but it certainly got the attention!