Welcome to Pherotalk. 
Yes, TUTH can be good in an established relationship but depending on other issues, Products like IH - to increase bonding/communication/closeness and the feelgood factor of Instant Shine can also be very applicable.
These products are also usable in much wider social situations and do not have the OD issues of very overt androstenone. Similarly, the more balanced product Ammo has its none content tamed by equal quantities of androstenol and androsterone, which makes it much wider in social application less likely to cause problems if you do the basic legwork to find your effective dose.
Yes, you can use your own cologne alongside pheros - simply layer them and give enough time for each layer to dry down before applying the next. The Instant Line and TUTH are all in Neroli - A light orange blossom scent that settles to background level after half an hour or so and complements many good colognes nicely.
Ammo comes in its own cologne scent - FF21, which is a subject of contention. Some love it, others hate/can’t tolerate it but it is very effective at diffusing the pheros. FF21 smalls similar to Green Irish Tweed but Ammo can be custom ordered in any other AD scent.
The only one you avoid mixing/layering is A314 in oil as this wrecks diffusion - keep its application points apart from any colognes/spray pheros. The A314 sprays are fine for layering though.
Have a good read of these threads:
Which cover all the basics of choosing and using pheros effectively.