Working out Dative Covalent Bonds in Lewis Structures - help!

Would greatly appreciate any help going. I'm self-taught.

Thought I had Lewis structures nailed but came across some holes in my understanding.

1. If there are formal charges on certain atoms in a Lewis structure and they can't be cancelled out by equal and opposite formal charges on other atoms (eg. -1 on an oxygen atom and +1 on a sulphur atom), does this mean that the molecule has a positive/negative charge overall and is an ion? It seems to work that way for the ammonium ion and sulphate ion (N has formal charge of +1 in the ammonium ion and two oxygen atoms have formal charge of -1 in the sulphate anion).

2. I'm working out formal charges using the formula - is it correct?
Formal charge of atom = No. of valence electrons in the atom - (No. of lone electrons + No. of bond pairs)

3. This is the real sticking point. I was doing well until meeting this question George Facer's exam questions:
Which does not contain a dative covalent bond?
PCl4+
NH4+
PCl6-
Al2Cl6

I already know that NH4+ and Al2Cl6 contain covalent bonds but I realised that my Lewis structure technique does not allow for working out which electrons come from which atom. The way I learnt, you simply share electrons out equally and don't worry about what electron originally belonged to each atom. After sharing them out equally, I work out formal charges using the equation above and try to get the numbers as low as possible - this usually works.

Now I understand, having looked at the answer, that it makes sense for PCl6- to have a dative covalent bond because P only has five electrons in its outer shell so can only form five normal covalent bonds. But I could also think of 'explanations' for why PCl4+ might have a covalent bond. I don't know any method to just work it out for sure, or a way to share out electrons that factors in the electrons in each atom's outer shell. So it seems like my system is seriously flawed.

If someone could help I would be incredibly grateful.