The gemara identifies 4 features of the menorah, and explains that none of them are essential (under certain circumstances). 1. Gold. 2. Kikar (quantity) of metal. 3. gevi'im, kaftorim and perachim 4. Hammered from a solid piece. The gemara says that the last three requirements are all dependent on the first. If made of gold, it must contain a kikar, must have gevi'im kaftorim and perachim, and must be hammered from a solid piece. But, if made from other metals, none of these requirements are necessary.
The Mishna L'melech (Beis HaBechira 3:4) in explaining the Rambam writes that the latter 3 requirements are contingent on it being gold, but when not of gold we treat each of these requirements slightly differently. The requirement of a kikar doesn't apply at all when made out of other metals. One can choose to use a kikar of silver, or more or less. The requirement of gevi'im kaftorim and perachim doesn't apply and is actually forbidden. One is not allowed to make these designs on any menorah made from materials other than gold (unlike rashi d.h. eina, who says that one doesn't need to put these designs into the menorah but is allowed to). Finally, the requirement of being hammered out from a solid piece, is preferable even when made from other materials but not essential. All these points are deduced from the language of the Rambam. On the first point of not requiring a kikar he derives from the language אין מקפידין על משקלה that it makes no difference. On the second point of gevi'im kaftorim and perachim he derives from the language אין עושין בה גביעים כפתורים ופרחים that one is not allowed to make them on a menorah of another material. The third point that it should be hammered out but doesn't need to be, is derived from the Rambam who says ואם היתה חלולה כשרה. The Kesef Mishneh explains that the Rambam means to say that it can be hollow, meaning that it need not be hammered out, but the Mishneh L'melech understands that ideally it should be hammered out even when made from other materials.
After all these halachos, the Rambam writes in halacha 5 - ואין עושין אותה לעולם מן הגרוטאות בין שהיתה של זהב בין שהיתה של שאר מיני מתכות. The mishneh l'melech understands that this is only lichatchila because the requirement of being hammered out is tantamount to the requirement of not being made from small pieces of metal. The source of not using גרוטאות is the word "miksha" which requires it to be hammered out. Since the Rambam holds that the requirement to hammer out is only lichatchila, so the problem with using small pieces of metal is only lichatchila. The approach of the mishna l'melech doesn't fit into the language of the Rambam. The Rambam seems to write clearly that even a menorah constructed of other material is passul if made from small pieces of metal. The Rambam seems to understand that the term מקשה has a double implication - 1. hammered, 2. not using multiple pieces. Although the gemara says that the din of מקשה only applies to gold, the Rambam understands that to refer to the requirement to hammer and therefore allows it to be hollow (which cannot be hammered). BUT the problem with making it from גרוטאות, small pieces of metal, which is derived from the term miksha, the Rambam considers to apply even to a menorah made from other metals.
The gemara 28b says that the menorah of chashmonaim was made of שפודים. Tosafos says that they weren't actually poles, probably because that would be a violation of making it out of multiple pieces of metal, just that they didn't have the gevi'im kaftorim and perachim since it wasn't from gold. The Ben Aryeh (back of gemara) says that the pesikta (2) impli8es that it was actually made of poles of metal. He explains that according to the Mishnah L'melech's understanding of the Rambam that using pieces of metal is only a problem lichatchila, the menorah of the chashmonaim may literally have been made from poles. However, according to my understanding of the Rambam that גרוטאות are passul, the Rambam would have to agree with Tosafos that the menorah of the chashmonaim wasn't actually made from שפודים. It was made from one piece just that it didn't have the gevi'im kaftorim and perachim.