"memakan" and "makan" are valid.
The difference between both is that: "makan" may be followed by an object and may not be. Whilst, "memakan" must be followed by an object.
(1) Saya makan.
(2) Saya makan roti.
(3) Saya memakan roti.
So, if we say "saya memakan", it is wrong.
My Indonesian grammar book explains that the short form doesn't have to refer to a concrete, specific instance of eating, whereas as the long form does. So, for example, the sentence 'Anda makan nasi ini' can mean 'you eat this rice' (meaning in general, this type of rice is a type of rice that you would eat) whereas the sentence 'Anda memakan nasi ini' can only mean 'you eat this rice (this rice specifically). This meaning is optional for 'makan'.
On other questions it was explain that you cannot mix saya and aku because that would be mixing formal with informal. I belive the same applies here. If a sentence has 2 verbs both should be formal or informal, but not one of each. Please, could a native speaker or an indonisian master confirm or denie this?
The languages of the Philippines and the languages of Indonesia (including Bahasa Indonesia) are all part of the same language family. So you get quite a few crossovers like this. It’s a bit like the relationship between English and German — some words pop up that are similar, but the languages usually seem very different