I am new to using a telescope and am having a hard time using a Barlow lens. I was able to find Jupiter multiple times using a 6mm lens with a telescope that has a D = 60mm and FL = 700mm. I am very close to seeing some detail (making out some color/features) but it is still a little too blurry. Whenever I equip the Barlow lens (x2) the object is nowhere to be seen (even when focusing and moving around). Any tips to using a Barlow? Is 6mm too small to use with a Barlow?
There are some rules of thumb when it comes to telescopes and usable magnification.
One is that your maximum usable magnification is two times the diameter of your telescope's aperture in millimetres (or 50 times the diameter in inches).
Another is that the best views will be provided by an eyepiece which has a focal length (in millimetres) equal to the focal ratio of your telescope.
(This one is basically saying "Use an eyepiece which gives an exit pupil of 1mm")
NOTE, however, that these are guidelines and not hard rules. Depending on the seeing where you live and the quality of the optics of your telescope and eyepiece, you can go above these 'limits'.
Think of them as hints to help people avoid spending money on eyepieces that they'll rarely be able to use :-)
Let's take your 60mm / 700mm telescope:
According to the first rule, 60 x 2 = 120x magnification is your theoretical maximum, and your 6mm eyepiece gives about that.
On good nights with exceptional seeing, you might be able to push the magnification higher, but probably not very often.
And the second, 700 / 60 = 11.6mm.
An eyepiece of this focal length will give an exit pupil of 1mm. An eyepiece of, say, 12mm in your telescope will show you all the detail you'll be able to discern. Going lower than this will give you a larger image, which can be useful, but you won't see any more detail. Most of the time you'll only just increase the amount of blur.
To answer your questions: "Any tips to using a Barlow? Is 6mm too small to use with a Barlow?"
Use a Barlow to give you focal lengths which you don't already have in an eyepiece.
Use a Barlow for those rare nights of good seeing, to push the magnification as high as possible.
Yes, 6mm + a 2x Barlow in your telescope results in too low a focal length and too high a magnification for all but the rarest occasions.
Eyepieces with focal lengths that low are for fast F-ratios - around F/4 and below - where they'll give usable exit pupils (and maybe usable magnification, too, given good seeing)
In your telescope, the 6mm eyepiece gives an exit pupil of half a millimetre and probably the highest magnification you'll ever be able to use.