Some have said you can't come on an M1 and then a J1, but I personally know quite a number of students who have done or are doing it right now.
And I'd say it's very likely myth perpetuated by some less knowledgable persons and maybe some fixed wing schools that want you to 'tied up' to them on J1 from the scratch.
Both rotary J1 issuing schools make no secrets about some students choosing to do PPL (H) on M1 and later (they both let me know their ideal and max TT and ratings for them to consider me for J1), with not too many hours TT get J1 so they 'save' 3-5 months of training on J1 towards instructing.
It's been discussed numerous times. It's a good thing if you want to make the most of it. I'll go on J1 in the future and starting my M1 in just under 2 months :P
RE: H269/S300 time building - well, California, Oregon and Washington state all have some schools with S300/269. Just look into it. Florida's got obvious advantages during European winter time..