You give a time phrase \"A couple of months ago\". This specifies a past time, so you must use the past tense. \"We started\" is past tense. It doesn't matter that the phrase isn't precise. It still refers to a time in the past.
If you don't use a time expression you could use either past tense or perfect tense. Both are correct. The past tense talks about a past action. The perfect tense \"We've started rehearsing again\" talks about the current state (as a result of the past action) But there is not much difference and both could be used.
\"We've started something special,\" Rankins told the team's senior reporter Eric Allen. \"Obviously, we didn't end the way we wanted to in terms of the record. We didn't show what we wanted to show throughout the season, but we've grown, showed promise and developed in some areas that needed to be developed. We still have some room to grow. That's the beauty of all this, that we still get a chance to work and grow toward something that we truly believe is capable of happening.\"
Until the early 2000s, the only known planets were located in our own neighbourhood, the Solar System. They broadly form two categories: the small rocky planets in the inner Solar System and the cold gaseous planets located in the outer part. With the discovery of exoplanets, planets orbiting stars other than the Sun, additional classes of planets were discovered and a new picture started to emerge. Our Solar System is by no means typical.