Yes as said you will need a sleeve / reducer depending on the exhaust you are looking at mating to the milly !
You never know though it might just fit ! If you specify to the manufacturer that you want to couple the your down pipe / cats to the standard cat back ?
Your system is designed to fit to the oem cat so your milltek tbo back will have the same connector as the oem cat back I'd imagine ? So if specify that you want your cats to run with the oem cat back your final fitting on the cats should then mate with your cat back I would of thought