techstocktrader 

Regarding my first question, I'm trying to figure out if I should change my commission plan with TradeStation from a 1¢ per share basis, up to 500 shares, to a fixed amount per trade. The fixed amount is $10 down to $5 depending on activity.

That will depend on your planned position size.

If you are normally trading 1000 shares or more then a $10 flat fee will be better than 1 cent per share. If your position size is less than 1000 shares or if you plan to scale out of trades in smaller lots then you should opt for the 1 cent per share offering.

Not being familiar with tradestation what is the minimum trade? Do the flat rates include exchange fees?

Regarding your question on lot size and charge per share versus lots;
In my opinion it would depend on the price of the stocks you trade. I prefer
$20-90 range.
Additionally, are you trading for nickels and dimes?.. or dollars.
Finally, regarding fills on 1000 share lots, personally, I only trade stocks
with 1,000,000 avg daily. tigher spreads between bids and asks w volume.
Also, if you trade too early while there are still tails on your candlesticks, then you run
a searious risk of getting crappy fills in and out, IMO.

deaddog 

That will depend on your planned position size. If you are normally trading 1000 shares or more then a $10 flat fee will be better than 1 cent per share. If your position size is less than 1000 shares or if you plan to scale out of trades in smaller lots then you should opt for the 1 cent per share offering. Not being familiar with tradestation what is the minimum trade? Do the flat rates include exchange fees?

Here are the 2 TradeStation Commission schedules. The per share schedule is basically 1¢/share up to 500 shares. The flat fee schedule starts at $10 up to 1000 shares and goes down to $5, depending on your number of trades.

Something interesting about TradeStation, you can only change your commission election or schedule once, no more:

"4. Can we change an equity account commission structure from flat fee to per share or vice versa?
We will allow you to change the commission structure in your equity account once, as a courtesy."