29.97 to 24 fps

I got a 24 proyect base, but some clips were shoot at 29.97 by accident, I'm asked to make them look like they're in 24 speed (since it looks in slow-motion).

Do I have to mess with speed or is it just a matter of rendering specs?

(There's no need of sync)

Are those shots interlaced? If not, just throw them into your 24 fps timeline and DR will adapt them with the algorithm you choose for re-timing in your project or in the Inspector.

I got a 24 proyect base, but some clips were shoot at 29.97 by accident, I'm asked to make them look like they're in 24 speed (since it looks in slow-motion).

That's a worst-case scenario, because the visual artifacts going from 29.97fps to 24fps are pretty severe, particularly in horizontal motion or pans. You could try doing a speed change down to 24fps with Optical Flow -> Enhanced Better and see if you can accept the picture quality. Otherwise, you could try Optical Flow -? Speedwarp, but it's very processor-intensive.

I had a similar scenario but mine was going from 23.97 to 18 fps as the director wanted to have the flashback part look like it was Super8. I tried re-timing it and had jerky results and interlacing issues. Finally I tried using the stop motion effect as suggested by Marc in another post, and with some experimenting and blending, it somewhat did the job.

It's bad case. I assume it's 29.97 progressive?
Use optical flow conversion as suggested and pray for the best.

You could also use Telestream Transform (pay per minute) service which uses Tachyon tech (which is good for low fps sources), but not sure how much better results you can get.

Is Tachyon better than Alchemist algorythm?

Sometime better, sometimes not.
Tachyon has better way of handling difficult bits- it has fallback feature, which will use simple algorithm (like blending) for areas which could produce too many artefacts and then blend that with rest of the frame.
I think for low fps files (up to 30p) it may be better.
Now free avs/vs tools also have advanced blending mode (and work with RIFE AI based tool). It may end up better than any of those 2. Have not tested it recently though, but they made big improvements over old mvtools based scripts.
Anything 50p/50i+ as source produces quite good results in all those tools. The lower source fps (with more motion blur) the harder to get good results.

I've recently tested TVAI speed changes vs DR's Speed Warp (4x slo-mo).
The results are close, but Speed Warp is faster on my hardware.