No.
You have no right to pick and choose halachos. It’s a mefureshe Gemara in Avoda Zara 7a:
??? ????, ??? ???? ???? ???? ??? ???? ???? ????, ?? ??? ??? ??? ???? ?????? ????? ?????? ??? ?????. ??? ??? ??? ??? ??????, ?’ ????? ?? ???? ???? ??? ???? ??? ??? ?????? ??? ?????? ??? ??? ?????. ?”? ???? ????? ???? ????? ?? ????
If someone is greater you follow the one who is greater, and if not, then if it’s d’oraisa you follow the machmir, and if it’s d’rabbanan you follow the meikil.
There is an exception, and that is the Gemara which says in Eruvin 6b:
???? ????? ???? ???? ??? ?????? ????? ????? ??? ???? ???? ????? ??? ??? ???? ????? ?”? ?????? ?”? ??? ?????? ?”? ??????? ?”? ???? ????? ???? (???? ?, ??) ????? ???? ???? ??? ?? ??”? ???????? ?????????? ?? ??”? ???????? ??????????
The specific application of the Gemara does not apply, as we know that a bas kol came out and paskened the halacha like Beis Hillel.
However, everyone understands that you may follow your rebbi’s opinion regardless of who agrees or disagrees with him, and that comes straight out of this Gemara. Since you are not picking and choosing, it is not hypocritical or wrong of you to follow your rebbi. If you would say “I’m going to be meikil like R’ Yaakov Emden over here” and “I am going to be meikil like the Noda biYehuda over here,” that is arbitrary and wrong according to both Gemaras.
These Gemaras refer to relying on someone. Each person who is at the level may come to a conclusion and pasken, even though others disagree, because that is his belief of the ???, and ??? triumphs all. But when one is not claiming to have figured out the truth, and certainly one who hasn’t learned a particular sugya in depth, is simply relying on others, in which case the above rules apply.
There are certain Poskim who were accepted by different segments of Klal Yisroel as “posek acharon.” The Shach, by the yeshivishe velt for example. That doesn’t mean we know he was bigger than the Taz. It means that he is our rav. And therefore in a case where we cannot figure out what the halacha should be we will take his word l’kula and l’chumra.
Some people hold the Mishna Brura is a posek acharon, or the Aruch Hashulchan, or the Maharsham. It’s the same thing. The rav that holds that the Aruch Hashulchan is a posek acharon will argue on him sometimes, but when he cannot be machri’a the halacha he will follow the Aruch Hashulchan l’kula and l’chumra.
If you wish to make R’ Yaakov Emden your rav and posek acharon, kol hakavod. But unless you follow him in every occasion that he disagrees with someone else and you aren’t capable of being machri’a not like him, you would be acting hypocritical, and you would be called a ??? according to the Gemara in Eiruvin.
This is why it is important to have a rav.
old man –
If he would have said he wants to follow all p’sakim of the Ya’avetz, I would have said yes.