If the creatures' tails were permanent, the most stable way for the creatures to walk would be on all fours, and quadrupedalism is somewhat less conducive for precision, which is necessary for scientific advancement.
However, if these creatures developed technology and modified their genetics to allow their tails to be extensions of their spine. Think about it, these creatures, in their normal mode, would be around 7 feet tall. When they go into combat mode, their tail would extend out of their body. This tail would not be weaponized yet, although some of the members of the species choose to have their tails spiked/weaponized.
At this point, their height is 4.5 feet, and they are automatically more agile dues to their decreased size. They now don their battle armor, some of the species attach blasters to their tails, some attach swords/blades, and others use prototype technology, which can allow for flight, stingers, etc. In this mode, they can go quadrupedal with weapons attached to their back and tails and claws on their limbs, or they can remain bipedal, and act as marksmen, infantry, horde control, with armors similar to Iron Man's, transforming suits that can make many types of weapons. Their tails acting as extra arms. They can also use their tails to jump and land.
A bit of anatomy
For those wondering how their tails retract and extend without affecting their legs, you can think of it as their spine/tail - the upper part, being attached to their head and arms, while their legs, hips, and pelvis are a separate part. If you were to look at their pelvis from the front, you would see a U-shaped hole, from the side you would see that it the opening to a canal that goes from the upper part of their pelvis to the lower part, the upper spine slides along this canal when it is retracting and extending, it remains attached to the canal via magnets. Their spine has powerful magnets embedded in it and their pelvic canal also has magnets embedded in it.
As for how the tail skin works. You can go about this two ways. The first way is simple - the creatures have hyper-elastic skin. The second way is probably more realistic. When inside the body, the tail is coated with a liquid that is kept as a liquid thanks to a special substance(gas) present in the creatures' bodies. When the tail is extended, it exits the body via a special opening near the small of the creatures' backs. When the liquid is exposed to air, it solidifies into a strong, tensile, nigh-impenetrable skin. The opening is also coated with this substance preventing the tail from retracting accidentally. All this happens in a fraction of a second. From the outside, however, the tail looks quite normal, the skin looking just like the rest of the creatures' skin. When the creatures want to retract their tails, their brains send out a signal that re-liquifies the skin and allows the tail to be retracted. Also, when the tail is retracted, the opening is covered with the hardened liquid.
Hope this helps.