What Ratio Is Good/Normal For Collecting/Iinvesting

There's no correct ratio. Everything is individual. The only ratio you should be looking at is the gold to silver to gold ratio in terms of price. It ranges from the 30s and 40s to the 60s and 80s. Right now it hovers between 63 and 65:1. Right now silver is "cheap" relative to gold, but when that numbers get to, say, 50:1 it would be time to either buy gold or sell silver to buy gold. Again, there's no correct way to do it; this is just what I've noticed over time. Palladium dealers are more scarce; you're going to run into liquidity problems even if palladium is a good choice. I would probably stay away from platinum but that's just how I feel. Industrial metals are remarkable, but only to people who can actually use them and manipulate them. People with coins/bars just hoard the metal for no reason at all other than tradition and fear.

Gold is absolutely easier to sell than Pt/Pd. That being said, they can still be sold relatively easily. I am a big fan of Pt group metals, and think they make just as much sense to hold as either gold or silver. I disagree with Libertad's assessment that industrial metals are held for no reason other than tradition or fear. They're held because of the fact that they are rare and useful, which is what makes them valuable.

As for your ratios, what you propose looks reasonable. I shoot for a 1:1:1:1 ratio in terms of value. That way, when ratios change, I'm buying what's cheaper relative to the others to keep my desired ratio.

Agreed. Silver and gold are highly liquid (any pawn or "we buy gold" store will take them...just know what they're worth beforehand).

Palladium and Platinum...not that easy. It is theoretically possible to walk into any pawn shop and sell them, but only if the pawnbroker knows what they are and wants to assume the hassle of finding a buyer for them. Other than that, I think I would only trust APMEX for selling those metals (some may disagree, but it's the same reason I distrust Gazelle.com and usell.com (etc) vs selling to Best Buy (Best Buy, like APMEX is big, conducts enormous amounts of business, and is more open publicly; Gazelle.com, usell.com, the same as other local or semi-local "we'll buy all kinds of precious metals" stores that prefer you to ship them the metal and they'll write you a check: stay away. They are small, conduct business but how much is private, and reviews online are pumped up by the companies themselves, if there are any reviews at all)

So, with that in mind and since APMEX locks in prices when you make your sale (regardless of what happens to the price of the metal when your products arrive to them), liquidity for Pt/Pd is better.

Other metals like rhodium on the other hand, Google searches provide sources to sell to, but how much you'd want to trust them is questionable, at best.

At what fineness?
For investment purposes it should be .999 or .9999