There's a command point quota you need to stay under. Building units will take points away from this quota, and destroying them (either through battle or decommissioning) will restore them. Campaign sometimes lets you exceed this quota through reinforcements, and you can exceed them at any time by capturing husks (e.g. destroyed Mammoth Tanks, Avatars). Most units use up 3 or 6 command points. For example, the Titan Mk II requires six CP, so under the Skirmish limit of 50 CP, you can build eight. Similarly, since an Engineer is only three CP, you can train sixteen of them. Defense has another quota for its structures, with Power, but it should be obvious you can't go over that number. You're free to mix and match at will, and is recommended, since the counter system in RA3 makes a return.
Also, taking out James is an exercise in frustration, since half of those damn AA turrets can't even reach, and the other two are guarded heavily by GDI. Not to mention it's more or less 4v1.