Just back from a week in Penang and Langkawi with our 3-month old.
I'd say just try it. Course it's different/more challenging with a tiny person in tow (not least cos of overprotective urges) but to be honest I think our Bean seemed happier in Penang's backpacker fleapits than the posher place we booked in Langkawi. But maybe he takes after his parents
Btw - I think every other guest had a small baby or child with them in Lk so you won't be alone if you do go - but we found being stuck on a posh resort a bit dull (and the sea had stingers in it!).
Breastfeeding was absolutely fine everywhere, I just took a VERY thin cotton sarong to cover him lightly with. Anything else and you're both sweltering. If we got hot I just found s-where with a fan and fed him, restaurants mainly, people were v sweet.
Heat might well be an issue - we live in Bangkok so our baby is used to it and sweltered away without complaint, but it seemed way hotter there. We all got a heat rash, but I just treated it with Calamine.
Dont think we could've done the trip without our Baby Bjorn Air carrier - lightweight, and if he bawled at bedtime his dad just took him for a stroll and he went straight to sleep.
We also took a thin pillow - sounds weird but acted as his bed (he slept in between us) and was perfect for the flights, bus trip and boat journey as he just snuggled into it and fed/slept. It had two pillow cases which acted like a sheet and meant at least one case was always clean.
Also - I have to say this and I know I'm biased - but have you considered Thailand? Malaysia was fine to travel with a tiny but I missed having a free babysitter (Thai waiting staff) at every meal and babies are treated like superstars here!
Give me a shout if you've queries on any more specifics.