Creating a seed by randomizing BIP 39 word list?

Howdy, fellow Ethereumists!

I want to make my own program to (offline) create a wallet (seed) by randomizing words from the BIP word list (I don't really trust any know source even if its open).

• Are there any restrictions or can I just use all the words?

• is it a security issue to use a randomizer in Java or Python?

• can I use a 12-word seed to also create a Bitcoin wallet? (I did this before, but does it always work?)

Thank you in advance!

/edit: - not enough entropy / possible flaws - generating real randomness (rdrand or real world dice) do the trick