I think there are several heuristics you can use:
Pronunciation
The pronunciation seems to me like it could go either way for Metalium, but not for Metallium, as I said in the comments beneath Dan's answer and Armen Ծիրունյան's answer .
The regular pronunciation for an English word spelled Metalium would indeed be with a "long a" /eɪ/. We would expect this due to a phenomenon called "CiV tensing": all vowel letters except for i tend to be pronounced long when followed by a single consonant, the vowel letter i, and another vowel letter. This is explained in the following chapter from Balogné Bérces Katalin's book The Pronunciation
of English: 12. Letter-to-sound rules – Part 2: Vowels.
However, there are some irregularly-pronounced words in English that lack CiV tensing, such as companion, valiant, and Italian. In addition, English speakers are generally not consciously aware of "rules" like this, so they may fail to apply them to newly derived terms. For example, the word nobelium is pronounced both as /noʊˈbiːlɪəm/ "no-BEE-lium" and /noʊˈbɛlɪəm/ "no-BELL-ium."
There are not many other words that end with the exact sequence of letters -"alium", and the ones that do exist have different pronunciations. The word dentalium is pronounced /dɛnˈteɪlɪəm/ "den-TAIL-i-um" according to the OED, following the rule of CiV tensing, but the brand name Valium is pronounced /ˈvalɪəm/ "VAL-i-um." In general, brand names are less likely to follow the regular sound-spelling correspondences that exist for the majority of normal English words.
The rule that vowels receive a "short" pronunciation before a doubled consonant letter such as "ll" is quite reliable, however, even for brand names. This is why I'd agree with Dan that the spelling Metallium makes the pronunciation more predictable.
Etymology
Setting aside the issue of pronunciation, a double ll is more faithful to the etymological source of the word metal, which comes from Greek metallon (μεταλλον) through Latin metallum. Because of this, we do use a double letter in English in some related words such as metallic and metalloid, which might be why it seems right to you. (For comparison, the word vocalic rhymes with metallic but is spelled with a single l because it comes from the Latin word vocalis, and the word petaloid rhymes with metalloid but is spelled with a single l because it comes from the Latin petalum.)
On the other hand, the words metallize/metallise/metalize/metalise and metalist/metallist can apparently be spelled with a single l or a double ll. This shows that some people do treat the English word metal, with one l, as a valid stem to which Latinate suffixes can be added.
There are many other words in English that seem to double the letter l before suffixes, such as crystallize, crystalline, coralline and sibylline (although dictionaries list both sibylic and sibyllic) and intervallic. In fact, the words crystal, coral, sibyl, and interval all come from words spelled in Latin with double ll. So from a historical perspective, this is not a case of single l being doubled before a prefix, but of double ll being simplified to l at the end of some words (specifically, l tends to be used instead of ll after an unstressed vowel).
What other people have done
I can't find any "real" words composed of one of these roots plus the suffix -ium. But I guess the next best thing is to look at other business names, brands and product names. In general, I found that neither variant seems obviously preferred.
From a Google search, Metallium Inc. is the name of a company that supplies samples of various elements, specializing in rare earth metals. "Metallium" is also used as the name of a weave produced by van Heek Textiles. There were also several results reflecting use in fantasy works and music titles and lyrics. On the other hand, "Metalium" is a German power metal band, a web HTML template by OTWthemes, and apparently a substitute for lead used when joining pipes.
"Crystallium" is used in the product name "Crystallium Wars TD" (an online game) and as a place name in the comic book "Saga of Crystar, Crystal Warrior." On the other hand, "Crystalium" has been used as the name of a French black metal band and as the romanization of a leveling system in the Final Fantasy game series.
"Corallium" is used as part of the scientific name of a precious coral, Corallium rubrum, and other corals in this genus. But "Coralium" is used in the scientific name of a sea snail and as the name of an ore in the AbyssalCraft mod to the game MineCraft.
It looks like different people often have different intuitions about what looks best in this situation.
