[RPG] Can a Cleric use Channel Energy to heal and damage at the same time

Yes as it states all creatures of one type are effected.

To target selectively a cleric should select the Selective Channeling feat which allows them to exclude targets up to their charisma modifier from the effect.

Clerics can also modify who they target using the Alignment channel feat to target outsiders instead.

Necromancy is not inherently evil

Necromancy spells are only characterized by their use of negative energy; negative energy is entropic in nature but is not a moral force. The inflict line of spells, for example, aren't evil.

But...

Necromancy does have some evil spells. Thankfully, they'll helpfully be labeled [Evil] in the spell descriptions!

Spells associated with particular alignments are indicated by the chaotic, evil, good, and lawful descriptors in their spell descriptions.

Per the Cleric class description, the Cleric cannot use spells with an alignment component opposed to their own; Good Clerics can't use [Evil] spells, Evil Clerics can't use [Good] ones. When in doubt, just check the spell's description!

Further, Channel Energy does not affect non-spontaneous spellcasting. While Channel Energy determines what spells a Cleric may cast without preparing, it doesn't affect spells that the Cleric memorizes ahead of time and does not otherwise restrict their casting list, though their alignment may do so (see above).