Your 401k should give you a list of what funds you can invest in - do that - put them in mutual funds - don't try and trade them yourself.
If you want to trade, then do that on the side with other money. Not with your 401k money. I am really not sure what you are talking about when you say I plan to keep 401k investments separate from stock investments - if you are in mutual funds, a good deal of them are stock funds. Some are bonds, but those are the more conservative funds and you are young so I would suggest you put more in the stock funds. Maybe you mean your trading account. That is fine. Put whatever you can afford to lose in that and trade and get a feel for it and you can even compare how well you are doing vs your 401k.
Usually when a company allows employees to buy stock at a cheaper price there are restrictions on when you can cash them. Some companies do matches - say for every 20 you buy they give you 5 - but you usually have to wait until they are vested to sell - probably every company has their own time period.
Usually when a company has a savings plan they have a representative you can talk to regarding how to allocate your money.
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