It is not a good idea to mix batteries in your car unless you plan on isolating them from each other. You can confirm this with Optima themselves. You will not find one single battery company that recommends mixing batteries. Why? Because it will cause them to fail prematurely. What does that mean to you? Well when you spend hundreds of dollars on a battery, you expect it (or should expect it) to last for many many years. This will only happen if you install it correctly, do not overcharge or over-discharge the battery, and keep it charged at all times.
If you have too small of an alt, you will not be able to keep the batteries charged, which will lead to a much shorter life of the battery. If you only upgrade the alt and do not have a big enough battery to assist the alt at times when it cannot respond to the demands of your system, then you will either fry an amp or the alt.
Your electrical system is comprised of three main crucial components that must work well together.
1) Alternator - creates all of the energy used in the vehicle, including powering your electronics and charging your batteries. However, at idle, some alts make very little or no power at all. They also cannot change their output levels of energy quickly. Stock alts only put out enough to cover the stock electrical, plus a very small amount of extra capacity for small accessories.
2) Battery(ies) - stores energy created by your alternator for times when the alternator is either not running or is incapable of reacting quickly enough to meet the demands of your electrical system. Proper batteries will also cushion the electrical demands of the system and will stabilize your voltage.
3) Wire - without the proper size wire to transfer the power made by the alt and stored by the batteries, your system will be worthless. You must use the proper sized wire to carry the load of your system throughout your car. The first upgrade should always begin with your wire, called the Big 3, and then progress to battery upgrades, then an HO alt, if needed.
