Stay in the tense

Is it right that if you start a sentence in the past tense, stay in that tense UNLESS you find it necessary to use a BUT or HOWEVER, which WOULD change the whole emphasis of the sentence from that point on and give you the freedom to do whatever you wish. SO i give an example here : She said she wanted to go shopping, BUT she haS to visit her sisters first. SO in this sentence, i started the sentence in the past tense,so i stay in this tense though the part after "she said" should be simple present tense, then i use comma and "But" to start the other part of the whole sentence, and i use the simple present tense. 1) How about if i use AND after comma, not BUT, do i still need to stay in the past tense ? Eg: She said she wanted to go shopping, and she prefers/prefered to hold a party for her sisters. 2)Then, if i do not use either BUT or AND after comma, I just say: She said she wanted to go shopping, she wants/ wanted to buy nice clothes. Do i need to stay in the tense or comma has the function which can let me change the tense, so that every time, when we use comma to separate, we can use the tense we want. A little bit complicated. But please try to understand and answer. Thanks ....