Entei isn't particularly viable in PvP. Since purifying can't be reversed, I would hold onto it as is until you're absolutely sure you want to use it.

Should I purify Shadow Pokemon?

When is purifying a Shadow Pokemon worth it? Many players often reiterate the line that purifying a Pokemon isn't worth it in almost any situation, but there are certainly situations where a purified Pokemon is more useful, even if it loses the 20% damage buff that comes with being a Shadow Pokemon.

Should I purify Shadow slowpoke?

You're not going to use them in battle and if you do purify it, it won't matter whether its CP is 20 or 600 as it'll end up the same, regardless. However, the IVs are important to look at. Shadow Pokemon are clearly an excellent opportunity to get high IV options, and if you're really lucky the coveted perfect IVs.