Is there a downside to not be all Renegade or Paragon?

As in many games with moral choice, the best way (game wise) is to go all out on either. This often puts you in a situation where you turn out to be either Mother Teresa or Vlad the Impaler. Staying somewhere in the middle locks you out of either bonuses. But at the same time it makes for more interesting role playing.

I'm playing through ME1 now and I'm going the Paragon route. Playing a nice Shepard is mostly rewarding (and it's the way I like him) but ever so often, I get into a situation where I really would like to slap faces. But because of the Charm/Intimidate skills I still choose the Paragon option.

This is getting really annoying because while a Paragon Shepard is a nice person to play, he also seem to be pushed around by everyone and their mum - i.e. he's a complete wuss.

Can I still receive full Charm while mixing in a few Renegade points when I feel like it?

As far as I know, maximum benefits for Paragon/Renegade are capped at 75%. So you can still get all benefits for being Paragon even if you take some Renegade points. The bonuses for each are as follows:

Paragon:

• 10% – Opens 2 Charm ranks. Gives 1 Charm point.
• 25% – Opens 2 Charm ranks. Gives 1 Charm point. Bonus: 10% shorter First Aid cooldown.
• 50% – No charm ranks/points. Bonus: 10% maximum health.
• 75% – Opens 2 Charm ranks. Gives 1 Charm point. Bonus: 5% reduction in cooldown on all powers.

Renegade:

• 10% – Opens 2 Intimidate ranks, gives 1 Intimidate point.
• 25% – Opens 2 Intimidate ranks, gives 1 Intimidate point, 10% reduction in weapon powers cooldown.
• 50% – 1 health regeneration per second.
• 75% – Achievement, opens 2 Intimidate ranks, gives 1 Intimidate point, 5% increase in damage/duration on all weapons and powers

Note that reaching 80% of either Paragon or Renegade grants an achievement. Having a high Paradon/Renegade score also grants a bonus when importing the character from Mass Effect to Mass Effect 2.