In a hybrid cryptosystem is safe to send the encrypted key together with the encrypted message?
Yes it is safe.
Assuming you get everything else right(Proper encryption mode), proper padding, secure random key, proper key management and avoid problematic other usages of same key, etc...
You have to provide the encrypted key to the legitimate user, if you have two different transfer systems with independent weaknesses you could transfer separately but that is probably rare and RSA applied properly (e.g OAEP) is sufficient so the encrypted key will reveal nothing about the key to that without the RSA private key.