If you have solid hardwood oak you can add the same species and plank size to your home at anytime = no problem. If you want to keep costs down and ONLY have the new solid oak hardwood finished at time of install, you can add a stain to it to make it "different". At a later date you have all hardwoods sanded and refinished in a matching staine/finish.

If you want to update your home with hardwood, and you want them all in the same colour, you can have your original hardwoods sanded and refinished at the same time as you install the new hardwood. This is the most common and most "professional" look you can achieve.

If you have engineered hardwood, then you will be 'forced' to look at a different product simply because engineered hardwoods are discontinued every 2 years. The likelihood of finding a perfect match is between slim and none.

If you MUST add in another species/colour into your home (because you have engineered hardwood) then many people will run the new colour in a different direction or on an angle to allow for a design "reason" for having 2 different hardwoods in one space/level.

In essence, having 2 hardwoods in one space is "done" but it isn't ideal. If you like it and you know you will live in the space for 10+ years, then go for it. It is your home.

I personally would look for a species and plank match...and then go from there. If you want 2 different colours then you can have areas stained while others stay natural...at least you will have the OPTION of returning everything to the same shade at a later date. You can never do that with 2 different species.